In portable and especially in hand-held radio devices the trend is to minimize the size of the device. Due to size requirements, internal antennas, i.e. antennas placed inside the cover of the device, have become more common.
Internal antennas are usually implemented using a planar inverted F-antenna (PIFA) structure. A PIFA antenna comprises a radiator plane and a ground plane, and some insulator between the planes, typically air. An RF signal is taken to the radiator plane through a radio signal feed. Both planes are connected to ground.
The performance of the antenna of a portable radio apparatus, such a as mobile phone, is sensitive to the objects nearby the phone. Objects near the antenna may cause losses in the transmitted and received signals. Thus, the mobile phone and the antenna are designed assuming that the phone is used in the most common way, i.e. held against the ear of the phone user. Traditionally the usage position of a phone is unambiguous, the user having a phone call always keeps the phone in the same position. This is due to the positions of the microphone and the speaker in the phone. To keep the body loss of the signal as small as possible, the antenna is designed in such a way that the antenna radiation pattern points away from the user.
However, as new different phone models are entering the market and the use of hands free equipment is increasing, the assumption regarding the traditional usage position of a mobile phone is no longer valid. For example, when hands free equipment is used the mobile phone can be situated in a pocket or on a table during a call. This can cause severe degradation in antenna performance due to increased body or object loss.